1. Field of the Invention
The present invention relates generally to the fields of gene expression, particularly tissue specific expression, and more particularly smooth muscle cell specific expression. The invention also relates to cell proliferation diseases such as atherosclerosis, restenosis following balloon angioplasty and airway blockage in asthma.
2. Description of the Related Art
The phenotypic plasticity of smooth muscle cells (SMCs) permits this muscle cell lineage to subserve diverse functions in multiple tissues including the arterial wall, uterus, respiratory, urinary and digestive tracts. In contrast to fast and slow skeletal muscle cells which fuse and terminally differentiate before expressing contractile protein isoforms, SMCs are capable of simultaneously proliferating and expressing a set of lineage-restricted proteins including myofibrillar isoforms, cell surface receptors and SMC-restricted enzymes. Moreover, in response to specific physiological and pathophysiological stimuli, SMCs can modulate their phenotype by down-regulating a set of contractile protein genes, and in so doing, convert from the so called "contractile phenotype" to a de-differentiated "secretory phenotype" (Mosse et al., 1985; Owens et al., 1986; Rovner et al., 1986; Taubman et al., 1987; Ueki et al., 1987; Belkin et al., 1988; Glukhova et al., 1988; Chaponnier et al., 1990; Gimona et al., 1990; Shanahan et al., 1993).
This phenotypic modulation has been implicated in the pathogenesis of a number of disease states including atherosclerosis and restenosis following coronary balloon angioplasty (Ross, 1986; Schwartz et al., 1986; Zanellato et al., 1990; Ross, 1993; Olson and Klein, 1994) and may also contribute to the airway remodeling seen in asthma (James et al., 1989). Restenosis following coronary balloon angioplasty is a major problem, and contributes to the 40% failure rate of this procedure (Schwartz, et al., 1992; Liu, et al., 1989). Restenosis occurs because the smooth muscle cells are stimulated to proliferate after angioplasty and thus block the arterial wall. Because of restenosis, balloon angioplasty is used mainly for palliation in patients who are not acceptable candidates for open heart surgery (Scientific American Medicine, Rubenstein and Federrnan, Eds., March 1993, Section 1, XII, page 11). A method is needed, therefore, to control or inhibit the proliferation of smooth muscle cells after angioplasty.
In addition, ample evidence demonstrates that airway smooth muscle contraction plays a critical role during acute episodic airflow obstruction in asthma (Knox, 1994; Rodger, 1992; Pueringer and Hunninghake, 1992; Black, 1991). Extra-muscular factors, including submucosal thickening (James et al., 1989), vascular engorgement (Lockhart et al., 1992), periadventitial inflammation (Ingram, 1991), or persistent airway closure with bronchial non-reopening (Gaver et al., 1990), may amplify lumenal narrowing during bronchial smooth muscle constriction. While these factors exacerbate airflow obstruction, it remains airway smooth muscle contraction that is ultimately responsible for the acute decrement of airway caliber. Prevention or reversal of muscular bronchoconstriction has therefore acquired a prominent role in asthma treatment. Because they inhibit force generation by airway smooth muscle, .beta..sub.2 -adrenergic agonists are recommended in recent NIH guidelines as "the medication of choice for treatment of acute exacerbations of asthma . . . " (National Asthma Education Program, 1991).
Yet, despite their obvious clinical utility, .beta..sub.2 -adrenergic agonists are not ideal medicines. Their chronic use has been associated with diminished control of asthma symptoms, due perhaps to receptor down-regulation (Tashkin et al., 1982), to enhanced constrictor hyperresponsiveness following cessation of regular .beta..sub.2 -adrenergic agonist use (Vathenen et al., 1988), or simply to masking of the underlying inflammatory process. Though controversial (Wanner, 1995), chronic use of potent .beta..sub.2 -adrenergic agonists might even increase asthma mortality (Crane et al., 1989). Furthermore, wide clinical and laboratory experience (Rossing et al., 1982) demonstrates that inhaled .beta..sub.2 -adrenergic agonists do not fully prevent acute airway narrowing in response to provocative stimuli. Together, these accumulated data indicate that: 1) inhibition of airway smooth muscle contraction does represent an important facet of the treatment of asthma, but 2) use of .beta..sub.2 -adrenergic agonists alone to achieve this goal is not the optimal solution.
Relatively little is understood about the molecular mechanisms that control SMC-specific gene expression. Only three smooth muscle cell specific genes have been studied intensively throughout development, SM.alpha.-actin, SM-myosin heavy chain and calponin-h1. However, of these three, SM.alpha.-actin and calponin-h1 are expressed in various tissues other than smooth muscle. It is also unfortunate that all three of the smooth muscle genes, SM.alpha.-actin, SM-myosin heavy chain and calponin-h1 are only expressed in quiescent vascular smooth muscle cells, and not in proliferating cells. Thus, there is still a need for discovery of a smooth muscle cell specific promoter that is not expressed in other types of cells and is constitutively expressed in both quiescent and proliferating cells.